Sewage sludge digestion process



Sept. 24, 1963 l.. @ENTER ET/u.' .3,105,041

sEwAGE sLuDGE: DIGEsTIoN PRocEss Filed Sept. 9. 1960 INVENTORS ATTORNEYUnited States Patent O 3,105,041 SEWAGE SLUDGE DlGESTlGN PRCESS AlbertL. Genter, Wyman Park Apts., Baltimore, Md., and Richard R. Kennedy andRobert M. Kennedy, both of 604 Mission St., San Francisco 5, Calif.

Filed Sept. 9, 1960, Ser. No. 54,918 1S Claims. (Cl. 210-6) Thisinvention relates to sewage treatment, and more particularly, to aprocess and system for anaerobic sewage digestion particularly adaptedto handle the waste materials of communities having a population ofapproximately 10,000 persons or less.

As explained in our U.S. Patent 2,893,957, issued luly 7, 1959, the timerequired for active anaerobic digestion of sewage can be denitelydiminished when: (l) the digesting mass is vigorously agitated to effectequal distribution of the partially and more completely digestedmaterials of the mass with inoculating seed; and (2) when there is afrequent pumpage of seeded, undigested material into the activedigesting mass. According to our aforementioned patent, vigorousagitation and frequent pumpage of seeded, undigested material into theactive digesting mass are accomplished by using a twostage digestionsystem wherein the rst stage includes at least two digestion units, andthe second stage includes at least one digestion unit. Such a two-stagesystem is completely satisfactory and operates yadvantageously in plantsfor disposing of waste materials of highly populated areas. 'Ihe systemtherein suggested is, however, not particularly well suited for lowpopulation waste material disposal plants, because it provides forutilizing a series of digestion units which necessarily are expensive tofabricate and install.

The present invention is directed to an improvement over the process andsystem disclosed in our-above-cited patent. More specically, the presentinvention has as one lof its primary objects the provision of a processand system of treating fresh sludges in a minimum of space and equipmentto convert the same into processed sludge suitable for delivery tosludge beds, which process and system provide for vigorous agitation of,and frequent pumpage of, seeded, undigested material into, the activemass of sludge being digested.

A still funther primary object of the present invention is to provide laprocess and system for treating fresh sludge conforming with thepreceding object and incorporating but one enclosed processing area ordigesting unit.

Still further, other, and more specific, objects of the presentinvention are: (1) to provide a process and system of treating freshsludge in a minimum of space and equipment to convert the same intoprocessed sludge suitable for delivery to sludge beds, as prescribedabove, wherein the system and process provide for mixing at leastpartially-processed sludge containing anaerobic bacteria with freshsludge to form an inoculated fresh sludge mixture with an alkaline pH;(2) to provide such a process of treating fresh sludge wherein thesludge mixture being treated is circulated through the processing areaor digesting unit while being heated and violently agitated; (3) toprovide such a process of treating fresh sludge wherein after heatingand agitation, the sludge mixture is diluted and allowed to remainquiescent in the processing area or digesting unit so that the sludgesolids separate from the sludge liquor to form a processed sludgemixture suitable for delivery to sludge beds; and (4) to provide such `aprocess of treating sludge wherein a controlled portion of processedsludge is withdrawn from the digestion area or unit and stored forre-introduction 3,l-5,ii4l Patented Sept. 24, 1963 thereinto and mixturewith a subsequent batch of fresh sludge.

The invention will be better understood and objects other than thosespeciiically set forth hereinabove Will become apparent to those ofordinary skill in the art when consideration is given to the followingdetailed description of the illustrative embodiments of the invention.To facilitate ya complete understanding of such embodiments, referenceis made to the annexed drawings, wherein:

FIGURE 1 is a schematic diagram and ow sheet presenting a system whichcan be operated in accordance with the present invention; and

FIGURE 2 is a schematic diagram and ilow sheet similar to FIGURE l, butshowing a system wherein processed sludge, such as that delivered fromthe system of FIGURE l, is further treated prior to delivery to thesludge beds.

To facilitate treatment of sewage, it is common to deliver the sewageinitially to a settling tank wherein the sewage solids are generallyseparated from the sewage liquids. As a result, one obtains (a) a freshsludge, comprising the sewage solids and some liquids, which is to beprocessed, and (b) some comparatively clear liquid which is delivered tothe settling tank with the untreated sewage, but which can be drainedolf to a suitable field. The present invention contemplates an initialseparation of the sewage to obtain such a sludge for further treatment,and thus, `as shown in FIGURE l, untreated sewage is passed through aconduit 2 into a settling tank 4. Preferably, the settling tank 4 is ofa circular coniiguration, and is provided with a centrally-disposediniluent well 6 provided with a series of T-shaped inlet heads 8 whichcancel ythe velocity of streams issuing from such inlet heads asexplained in our co-pen-ding application Serial No. 825,442, filed July7, 1959, -now U.S. Patent 3,067,878. The settling tank 4 is 4alsoprovided with a. sludge collector or sweeper itl having veins extendingadjacent .the bottom of tapered Wall 12 of the tank 4 to move settledsludge ltoward the tank outlet 14. Such construction is also describedin our aforestated co-pending application. Other publications discloseadequate settling tanks, however. While we have explained a particulartype of settling tank above, and while such a settling tank is used inaccordance with the preferred embodiments of the invention, it should beunderstood that any suitable settling tank may be used to separate -afresh sewage sludge from untreated sewage for further processing.Regardless of the tank used, however, normally the sludge settles to thebottom of the tank, and the clear sewage liquids overiiow into a laundersuch as that designated by numeral 16 in 'FIGURE 1, and are then drainedoff for desired disposition through a conduit such as that designated bynumeral 18.

The fresh sludge which is delivered to the outlet 14 of settling tank 4passes into a conduit 20 having a valve 22 therein which serves -todirect the fresh sludge into either conduit 24 leading directly into theprocessing area 26, or lto the conduit 2S which leads directly to theinput of a sludge mixer 30. Preferably, the conduit 24 includes a valve32 which is normally maintained closed, and preferably the valve 22 isnormally maintained open so that fresh sludge is passed directly to thesludge mixer 30.

The sludge mixer 3b, or as it may be termed, the sludge receiving anddistributing unit, contains partially processed sludge with anaerobicbacteria therein, and serves to mix such processed sludge with the freshsludge entering through conduit 28 to provide a fresh sludge mixture.

Preferably, two partsV of fresh sludge are mixed with one part of atleast partially ydigested sludge in the mixer 30, and the mixture ismaintained with an alkaline pH. To maintain the alkalinity of themixture, milk of lime or any other suitable alkaline substitute may beadded through the conduit 32 leading into the mixer Sil.

The fresh sludge and the partially-processed sludge pass throughanoutlet conduit 34 and a pump inlet conduit 36 to a circulating pump 38`which pumps the mixture through a conduit 4u, heater 42, and inlet line44 into the enclosed processing area or digesting unit 26. For suchoperation, the valve 46 in conduit 34, the valve 48 in conduit 44', andthe valve Sii in conduit 44 are maintained open, and the valve 52 inconduit 54 and the valve "56 in conduit'SS are maintained in closedposition.

After the processing area or digesting unit 26 has been filled to asuitable level, the valve 46 in the outlet conduit of the mixer 3) isclosed and the valve S2 in the outlet yconduit 54 of the digesting unit26 is opened. During all of the aforementioned operations, the valve6din the drain-olf conduit 62 Yis closed and the valve 64 in thedrain-olf conduit `66 is closed.

VFrom the foregoing description, it should be apparent that the initialsteps of the process provided by the invention Vare conducting freshsludge to a unitary enclosed processing area and introducing at leastpartially-processed sludge containing anaerobic bacteria into theprocessing area to forman inoculated fresh sludge mix-ture with analkaline pH in such area. It should also be apparent that according tothe preferred operation described above, the fresh sludge is mixed withthe partially-'processed sludge in a mixer or sludge receiving anddistributing unit 3@ prior to conduction of the fresh sludge, andintroducing of the at least partially-processed sludge, within theprocessing area or digestion unit 26.

After the inoculated fresh sludge mixture, or a suitable amount thereof,has been placed in the digestion unit 26, and after valve 46 and valve52 have been closed and opened respectively, then circulating pump 3Sserves to continuously circulate the inoculated fresh sludge mixturethrough the Iprocessing area or digesting unit 26 and also servesthereby to agitate such mixture. It will be noted that, as suggestedabove, a heating funit 42 is provided in the circulating path extendingthrough conduit 54, conduit 36, circulating pump 3S, conduit 4i?,conduit 44', and conduit 44. The heating unit serves to maintain thetemperature of the inoculated fresh sludge mixture at a suitable levelfor anaerobic bacteriological action, i.e., at a temperature from about20 C. to 60 C.

Although the recirculating pump 33 provides for some severe agitation ofthe inoculated fresh sludge mixture, preferably agitation of suchmixture is further stimulated by introducing compressed gas into theenclosed processing area or digestion unit 26 through a variable-speedgas blower 68 disposed centrally of, and near the bottom 70 of theenclosed area or digestion tank 26. Compressed gas is fed to the blower68 through a conduit '72 extending from a compressed gas source 74.

AV oating gas `dome 76 is provided at the upper end of the processingarea or digesting unit 26 and a gas outletY conduit '78 extendstherefrom to the compressed gas source 74 to feed gases exiting from thetop of the unit 26 to the compression source. A valve 8G is provided inthe conduit 78, and by adjustment of the valve Sil the ow of compressedgas through the digestion unit, and thereby the `degree of agitation ofthe sludge mixture within the unit, can be controlled.

It has been found that in most instances during violent agitation of themixture a froth collects at the top of the sludge mixture undergoingtreatment in the processing area or digesting unit 26 due to the activedigestion. Such froth may be dispersed and forced to yield its floatedsolids by means of water sprays issuing from nozzles such as thoseschematically presented in FIGURE l and designated by numerals S2 and84. A suitable water line S6 4 v leads to such nozzles, and is providedwith a valve 9i) so that the sprays may be cut on or olii as desired.

During recirculation and processing of the sludge mixture, alkalineconditions should be maintained at all times. For this purpose, assuggested above, a suitable alkaline chemical solution may |beintroduced into the mixer through the conduit 32, and the valve 46 can-be opened to allow the solution so introduced to pass into the mixturecirculating through conduit 54 by virtue of the pumping action of the:recirculating pump 38.

After the sludge has been at least partially processed by continuousrecirculation as described above, then the same is diluted by passing-Water into the mixture'through the conduit S6 and spray nozzles 82 and84. The mixt-ure which is partially treated sludge, consists of a sludgeliquor and sludge solids, and the water which is introduced is purerthan the sludge liquor and serves to dilute the partially-treated sludgemixture. After the mixture has been suli'iciently diluted, the valve gt!in the water line y86 is closed, and either then, or preferablypreviously, the recirculation pump- 38 is stopped and thepartially-treated and diluted sludge mixture is allowed to remainquiescent in the enclosed processing area or digestion unit'26 wherebythe sludge solids of the mixture separate from the diluted sludgeliquor, and aprocessed sludge mixture exists in the processing area ordigestion unit 26 which is suitable for delivery to sludge beds.

When the sludge mixture undergoing treatment has remained quiescent forseparation as desired, then valve 43 in conduit 44', and valve 56 isconduit 5S are closed, md valve '6u Iin drain-off line 62 is opened. Thevalve 50 having previously ibeen opened ydoes not interfere withdrain-off of the processed sludge liquors through conduit 62 and thus byvirtue ofthe valve operation described immediately above, the processedsludge liquors are passed Vto a suitable drain ield. Y

Either simultaneously therewith, or after the sludge liquor has been:drained o, the valve 64in the drain-off line 66 is opened, and thevalve 46 in conduit 34 and valve 4S in conduit l44 are closed, and thesludge solids are drained out of the processing area ordigestion unit 26through conduit v66. If necessary, the circulating pump 38 can be usedto pump the solids out of the digestion unit 26.

It is important to understand that all of the processed sludge is notdrained off according to the invention. lnstead, atleast a portion ofthe processed sludge is stored for subsequent mixing with fresh sludgeto be treated.

ln the system shown in FIGURE l, a liquid portion of the processedsludge may be stored in the mixer 36 by closing valves 6i) and 48 andopening valves 5t) and 56 whereby the processed sludge liquor drainsinto the mixer. To store a portion of the solid processed sludge, thevalve 50 is closed and the valve 46 is closed, but the valves 52, 43 and56 are opened so that the circulating pump 3d can be activated to pumpthe processed solid sludge into the mixer 36 through the inlet conduit5S leading thereto.

Preferably the mixer 30 is provided with an overow Conduit 92 so that iftoo much liquid enters the mixer, the same will overflow in much thesame manner as liquid overflows from the launder 16 of the settling tank4 described initially in connection with FIGURE l. Y By again referringto FIGURE l, it will be noted that while the processing of the sludge iscarried out in the processing area or digestion unit 26, untreatedsewage can be conducted to the settling tank 4 and allowed to settletherein, so that after processing of one batch of sludge is completed inthe areaV or unit 26, another batch of sludge can be immediately passedthereto for processing. Of course, for this purpose, the valves 22 and32 are maintained closed during the timev that some sewage is settlingin the tank 4 and other sewage is being treated in t'ne unit 26.

.While it has been assumed hereinabove that the process is initiated byconducting the fresh sewage through the conduit 28 to the inlet of themixer 30, it should be understood that the provision of a mixer 30,although preferred, can be eliminated. More specifically, in instanceswhere cost is a primary factor ydue to the small population, the mixer30 can be replaced or eliminated, and fresh sewage can be fed directlyfrom a suitable settling tank such as that designated by numeral 4,through an inlet conduit extending directly into the digestion unit orprocessing area 26. More particularly, a conduit such as that designatedby numeral 24 and provided with a valve 32 can be used for this purpose.A certain predetermined or controlled amount of processed sludge can beleft in the bottom of the processing area or digestion unit 26, and ifthe mixer 3Q is eliminated, then mixing of the partially-processedsludge with the fresh sludge takes place during circulation of themixture as described above and during agitation of the same, as alsodescribed above. In other words, if desired, the mixing of the freshsludge with the processed sludge can take place in the digesting unititself, however, it should be understood that the provision of a mixeris preferred since it provides for greater uniformity of distribution ofthe processed sludge throughout the fresh sludge-processed sludgemixture.

Still another modiiied form of the invention provides for replacing themixer 30 by a storage tank or other storing means whereby a portion ofthe processed sludge can be maintained separate from the .digesting unitor processing area 26. According to this modiiication, fresh sludge isconducted directly into the digestion unit or processing area 26, and asthe sludge is being passed into such area or unit, the circulating pump3S serves to gradually pump processed sludge from the storage meansprovided into the digestion unit or area 26. .This system is somewhatmore eicient than merely leaving part of the processed sludge in thedigestion unit and then passing fresh sludge therein, but, of course, isnot as eicient as using a mmer 30.

From the foregoing discussion it should be apparent that the sludgebeing processed is mixed more or less on a continuous basis exterior ofthe digestion unit or processing area and is maintained active withinthe digestion unit or area during violent agitation thereof. Thisaccomplishes three principal activities, all of which contribute tokeeping the biological factors operating at peak capacity at all times.First, the active organisms are kept continuously in contact with thefood supply; second, the food supply is uniformly distributed and madeavailable to all of the active digestion organisms possible; and third,the concentration of the inhibitory biologicd intermediates and endproducts are maintained at a minimum level. All of these factors keepthe working population of active organisms and their food supplycooperating at peak eiiiciency.

In addition to the advantages of the present invention set forthhereinabove, it should be pointed out that a further advantage lies inthe provision of a system adapted to handle a higher percentage ofsolids than systems heretofore available. Authoritative investigationsupported by the Public Health Service research grants (ii-75) statesthat mixing of high-rate digesters becomes a problem as the total solidscontent of the mixed liquor approaches 6 percent. This is true ofcertain prior units. The basis for the percentage lies in the fact thatthe more volatile matter present in any sewage sludge, the less itsspeciiic gravity becomes and the more occulent and more jelly-like itbecomes. This quite evidently means that the sludge absorbs and adsorbsmore water. In other words, with lower ash content and more volatileorganic content, e.g., 35 percent xed solids and 65 percent volatilematter, the voluminous nature of the solid portion of the sludge becomesso great that the solids are exceedingly difficult to settle undergravity. With the employment of sludge activation by aeration, thisilocair and gases.

culent nature of the solids becomes more absorbent for However, thislimitation is readily overcome by the present invention which usesunaerated washing of the digested solids by relatively gas-free waterthrough removing gas and entrained air from the solids. Because of this,the present invention provides for readily increasing the solid contentof the sludge recirculated through the digester system. Even if thesettling tank 4 is used -for collecting secondary sludge solids of highVolatile content prior to digestion, the return of elutriated digestedsolids to the mixer 30 -for incubation purposes of the fresh sludge fedthereto lfrom the tank 4, the recirculated elutriated digested sludgemay easily contain 8 percent solids of higher moisture content and loweralkalinity than those in the report mentioned above. Elutriationincreases the solids in any sludge and obviously adding such elutriatedsludges back to digestion of fresh sludge will increase the solids inthe fresh sludge and result in an increased percentage of solids in thesludge being processed.

Although the process of treating sludge described hereinabove iscompletely satisfactory in many instances and desirable for use in smallpopulation areas, by a further minimum expenditure, the system can bemade somewhat more eicient. Particular reference is made to FIGURE 2,wherein the system of FIGURE 1 is shown in block form as coupled with anauxiliary outlet settling tank generally designated by the numeral 10).Preferably such tank is constructed in accordance with our applicationSerial No. 813,231, now Patent No. 2,973,866. Generally, such a settlingtank includes a sloped bottom wall 102, and is provided with a sludgeconveyor such as that schematically shown and designated by the numeral104 which serves to convey settled sludge to an outlet 106 of the tank100. In addition, the tank includes an inlet well 108 from which extenda plurality of T-shape distributing heads 110 which direct incomingstreams against one another to cancel the inlet velocity of suchstreams. Of course, an overow launder such as that designated by numeral114 is provided so that the overflowing clear liquid can be drainedthrough a suitable conduit such as 116 and returned to the plantinfluent 2 of FIGURE 1 or to the euent 18.

Now, according to the present invention, a tank such as that designatedby numeral lili) is coupled with the system of FIGURE 1 by connectingthe drain line 66 with the inlet Well 103 of the settling tank 169 bymeans of a suitable conduit such as Ithat designated by numeral 118.Also, a pump such as that designated by numeral 121i -is connected in anoutlet conduit 122 extending from the outlet well 166 `of the tank 10i?.The pump 126 is adapted to pump solids which have settled in the ltank169 back to the mixer 30 of FIGURE l through la suitable conduit such asthat designated by numeral 124i.

The pump conduit 122 leads into a valve 125 which may be lof thecommonly used three-way type, ie., a valve lwhich will allow material topass -therethrough in one Iof two selectable positions and Iwhich willprevent passage of material therethrough in -a third position. Byproviding a valve such as that schematically shown and designated bynumeral 126, the settled solids of tank may be either pumped back to themixer 30 or pumped out through a suitable conduit 128 to 1a drain field.yOf course, the valve 126 can be closed so that no material will bepumped therethrough.

When the improved system oi' FIGURE 2 is used, then there is no need toleave the processed sludge mixture in the processing area or digestionunit 26 of FlGURE l to settle the solids of the processed sludge fromlthe liquors thereof. Instead, after :the sludge undergoing treatmenthas been suitably processed, and is ready for such settling, it can bedrained Ithrough the outlet conduit 66 and conduit 118 lcoupled theretofrom the processing area or digesting unit 26 to the settling tank orarea 100.

Moreover, instead of diverting a portion of the processed sludge leavingthe digestion unit or processing area 26 into the mixer 3d, theprocessed sludge can all be .passed `to the settling tank lill?, andthen the controlled portion of the processed sludge to be returned formixture with fresh sludge can be pumped by pump l2@ through conduitT122., valve izo, :and conduit 12d back to the mixer 39. The system ofFIGURE 2 oan thus handle a larger volume of waste, and accordingly hasadvantages over the system of FGURE l in the somewhat more denselypopulated areas.

ln certain instances it Amay be desired to heat the processed sludgebeing returned through `conduit lf2/3v .the 'circulating system, itshould be understood that lheater l2 can be elsewhere located, as forexample in conduit S4. The important factor is that the circulatingsludge is suit-ably hea-ted for desired anaerobic bacteriologicalaction.

Similarly, pump 3S can be coupled in the system in -a location otherthan that shown, however, preferably pump 3S is located so as to pumpsludge out of drain conduit 66, as well through the unit 26counter-clockwise as shown, and counter-current to the flow ofcompressed gas entering the vent 26 through blower 68.

G-ne 4additional factor, not mentioned hereinabove, 1s that, if desired,a conduit such .as that designated by vntuneral 73 and having a valve 75therein, can be provided to feed compressed gas to conduit 24. Such gaswhen fed by `opening valve 75 aids movement of sludge through conduit25.- and provides for agitation of such sludge. This is of particularimportance when the modied system of the invention eliminating mixer 3dis used.

While the foregoing description sets forth return of processed sludge tomixer 3% at the end of processing each batch of Vsludge being treated,it should be understood that the return may be intermittent duringtreatment of each batch by :adjusting the valves for return as desired,or the return can be made continuous by adjusting valve 56 so that acontrolled amount of sludge is passed to mixer El@ during eachcirculation of a batch being treated Aas described.

After reading the foregoing detailed description of the invention, itshould be apparent that the objects set forth at the outset of thisspecification have been successfully achieved. Undoubtedly, 'afterreading the foregoing description, various modifications will becomeapparent to those of ordinary skill in the art, which modioations fallIwithin the scope and spirit of the invention.

Y What is claimed is:

l. ln a process of treating fresh sludges in a minimum of space andequipment to convert the same into processed sludge suitable fordelivery to sludge beds, the steps of (l) conducting fresh sludge to asingle enclosed digestion unit; (2) introducing at leastpartially-processed sludge containing `anaerobic bacteria into said unitto form an `inoculated fresh sludge mixture with an alkaline pl-l insaid unit; (3) then circulating said fresh sludge mixture through saidunit while heating said fresh sludge mixture and violently agitatingsaid fresh sludge mixture to form a treated sludge mixture includingsludge liquor and sludge solids; (4) then stopping said circulating andagitating and diluting said treated sludge mixture lin said unit withwater purer than said sludge liquor; (5) then allowing saidpartially-treated Yand diluted sludge mixture to remain quiescent invsaid unit whereby said sludge solids separate from said diluted sludgeliquor to form a processed sludge mixture suitable for delivery tosludge QJ beds; (6) withdrawing at least a portion `of said processedsludge mixture from said unit; 'and (7) lwithdrawing a controlledportion of said processed sludge mixture from said unit and storing thesame for reintroduction into said unit.

2. ln a process of treating fresh sludges in a minimum of space andequipment to convert the same into processed sludge suitable Ifordelivery to sludge beds, the steps defined in claim l and furtherincluding the step of adding an alkaline solution to said circulatingsludge to maintain an alkaline pH in said unit.

3. ln a process of treating fresh sludges in a minimum of space andequipment to convert the same into processed sludge suitable for`delivery to sludge beds, the steps defined in claim 2 wherein saidinoculated fresh sludge mixture comprises two parts of fresh sludge toone part of at least partially-processed sludge.

4. ln a process of treating fresh sludges in a minimum of space andequipment to convert the same into processed sludge suitable fordelivery to sludge beds, the steps defined in claim 1 wherein saidinoculated fresh sludge mixture comprises two parts of fresh sludge toone part of at least partially-processed sludge.

5. In a process of treating fresh sludges in a minimum of space andequipment to convert Ithe same into processed sludge suitable fordelivery to sludge beds, the steps defined in claim l wherein saidheating of said fresh sludge mixture maintains the same between 20 C.and 60 C.

6. ln a process of treating fresh sludges in a minimum of space andequipment to yconvert the same -into processed sludge suitable fordelivery to sludge beds, the steps defined in claim 5 and furtherincluding the step of passing compressed gas through said unitcountercurrent to the circulation of sludge therethrough to aid saidagitation.

7. `In a process of treating fresh sludges -in a minimum of space andequipment to convert the same into processed sludge suitable for`delivery to sludge beds, the steps A. efned in claim l and furtherincluding the step of passlng compressed gas through said unitcountercurrent to the circulation of sludge therethrough to aid saidagitation.

y8. -In a process of treating fresh sludges in a minimum Or" space andequipment to convert the same into processed sludge suitable fordelivery to sludge beds, the steps defined in vclaim 7 and furtherincluding the step of spraying said circulating sludge to destroy frothaccumulations thereon.

9. In a process Vof treating fresh sludges in a minimum Of space andequipment to convert the same into processed sludge suitable fordelivery to sludge beds, the steps defined in claim l and furtherincluding the step of spraying said circulating sludge to destroy frothaccumulations thereon.

10. In a process of treating fresh sludges in a minimum of space andequipment to convert the same into processed sludge suitable fordelivery to sludge beds, the steps defined in claim 9 and furtherincluding the step of adding an alkaline solution to said circulatingsludge -to maintain an alkaline pH in said unit.

l1. In a process of treating fresh sludges to convert the same intoprocessed sludge suitable for delivery to sludge beds, the steps ofconducting fresh sludge to a mixing station; mixing said fresh sludgewith partiallyprocessed sludge containing anaerobic bacteria to forni aninoculated fresh sludge mixture; conducting said inoculated fresh sludgemixture to a single enclosed `digestion unit; .circulating saidinoculated freslrsludge mmture through'said unit while heating saidinoculated fresh sludge mixture and violently agitating said freshsludge mixture to form a treated sludge mixture including sludge liquorand sludge solids; then diluting said treated sludge mixture in saidunit with water purer than said sludge liquor; then allowing saidtreated sludge mixture to remain quiescent in said unit whereby saidsludge solids separate from said diluted sludge liquor to form aprocessed sludge mixture suitable for delivery to sludge beds; andconducting a controlled portion of said processed sludge mixture to saidmixing station for mixing with fresh sludge.

12. In a process of treating fresh sludges in a rninimum of space andequipment to :convert the same into processed sludge suitable fordelivery to sludge beds, the steps defined in claim 11 and furtherincluding the step of adding an alkaline solution to said circulatingsludge to maintain an alkaline pH in said unit.

13. In a process of treating fresh sludges in a minimum of space andequipment to convert the same into processed sludge suitable forydelivery to sludge beds, the steps dened in claim 11 wherein saidheating of said fresh sludge mixture maintains the same between 20 C.and 60 C.; and further including the step of passing compressed gasthrough said area countercurrent to the circulation of sludgetherethrough to aid said agitation.

14. In a process of treating fresh sludges in a minimum of space andequipment to convert the same into processed sludge sui-table fordelivery to sludge beds, the steps defined in claim 111 and furtherincluding the step of spraying said circulating sludge to destroy frothaccumulations thereon and wherein said inoculated fresh sludge mixturecomprises two parts of fresh sludge to one part of at leastpartially-processed sludge.

15. In a process of treating fresh sludges in a minimum of space andequipment to convert the same intok processed sludge suitable fordelivery to sludge beds, the steps defined in claim 11 wherein saidinoculated fresh sludge mixture comprises two parts of fresh sludge toone part of at least partially-processed sludge.

References Cited in the file of this patent UNITED STATES PATENTS2,188,847 Streander Jan. 30, 1940 2,777,815 Forrest Ian. l5, 19572,786,025 Lamb et a1 Mar. 19, 1957 2,893,957 Genter et al. July 7, 1959

1. IN A PROCESS OF TREATING FRESH SLUDGES IN A MINIMUM OF SPACE ANDEQUIPMENT TO CONVERT THE SAME INTO PROESSED SLUDGE SUITABLE FOR DELIVERYTO SLUDGE BEDS, THE STEPS OF (1) CONDUCTING FRESH SLUDGE TO A SINGLEENCLOSED DIGESTION UNIT; (2) INTRODUCING AT LEAST PARTIALLY-PROCESSEDSLUDGE CONTAINING ANAEROBIC BACTERIA INTO SAID UNIT TO FORM ANINOCULATED FRESH SLUDGE MIXTURE WITH AN ALKALINE PH IN SAID UNIT; (3)THEN CIRCULATING SAID FRESH SLUDGE MIXTURE THROUGH SAID UNIT WHILEHEATING SAID FRESH SLUDGE MIXTURE AND VIOLENTLY AGIATING SAID FRESHSLUDGE MIXTURE TO FORM A TREATED SLUDGE MIXTURE INCLUDING SLUDGE LIQUORAND SLUDGE SOLIDS; (4) THEN STOPING SAID CIRCULATING AND AGITATING ANDDILUTING SAID TREATED SLUDGE MIXTURE IN SAID UNIT WITH WATER PURER THANSAID SLUDGE LIQUOR; (5) THEN ALOWING SAID PARTIALLY-TREATED AND DILUTEDSLUDGE MIXTURE TO REMAIN QUIESCENT IN SAID UNIT WHEREBY SAID SLUDGESOLIDS SEPARATE FROM SAID DILUETED SLUDGE LIQUOR TO FORM A PROCESSEDSLUDGE MIXTURE SUITABLE FOR DELIVERY TO SLUDGE BEDS; (6) WITHDRAWING ATLAST A PORTION OF SAID PROCESSED SLUDGE MIXTURE FROM SAID UNIT; AND (7)WITHDRAWING A CONTROLLED PORTION OF SAID PROCESSED SLUDGE MIXTURE FROMSAID UNIT AND STORING THE SAME FOR REINTRODUCTION INTO SAID UNIT.